The technology to improve the response of the power supply apparatus is described, for example in U.S. Pat. No. 5,844,403 and U.S. Pat. No. 5,583,752. However, because the technology described in the aforementioned patents assumes that an imaginary-number type PID (P: Proportional element; I: Integral element; D: Differential element) control is used, there are some problems that it is difficult to design a control circuit, and when the control circuit is realized, the number of necessary components becomes large.